As is well known, the demand for magnetic field sensors for mobile devices, such as mobile phones, personal navigation devices, smart watches, etc., to implement electronic compass applications, such as providing step-by-step directions, has been increasing rapidly as GPS functionality is now found in many of these devices.
There are many different types of magnetic field sensors and those based on anisotropic magnetoresistive (AMR) technology are also well known. An AMR sensor operates on the principle that there are some materials, e.g., permalloy, that exhibit a resistance that is dependent on the orientation of the magnetization of the material. In most AMR materials, the resistance of the material is highest when the current in the material is running parallel with the magnetization vector as shown in FIG. 1a. Examples of such technology are described in U.S. Pat. Nos. 4,847,584; 5,247,278; 5,521,501; 5,952,825 and 8,525,514, each of which is herein incorporated by reference for all purposes.
In an AMR device, a thin film permalloy material is deposited on a silicon wafer while a strong magnetic field is applied to create permalloy resistors. The magnetic domains of these permalloy resistors are aligned in the same direction as the applied field thereby establishing a magnetization vector. Subsequent lithographic and etching steps define the geometry of the AMR resistors.
In a well-designed AMR sensor, the magnetic material is designed to have a preferential magnetization orientation referred to as the “easy axis” of the material. As a result, under a zero magnetic field, the material will be magnetized along the easy axis. As a magnetic field is introduced, however, the magnetization of the material will rotate toward the external magnetic field. The stronger the field, the more the magnetization will rotate away from the easy axis as shown in FIG. 1b. 
Magnetic field sensors are designed to operate over a defined full-scale range. The full-scale range of a sensor is the range of values between the minimum and the maximum values that the sensor can accurately measure. In an ideal sensor, if the input stimulus is outside of the full-scale range, the output of the sensor clips to the max and min of the full-scale range, as shown in FIG. 2. In many instances, this clipping is perceived as being beneficial because it provides information to indicate that the sensor is saturated and, therefore, its measurement outputs are suspect.
When the external magnetic field becomes higher than the saturation field of AMR material, however, the resistance is no longer a function of the field strength and is instead only a function of the field orientation. As a result, AMR sensors do not clip when the measured magnetic field exceeds the full-scale range. Instead, the sensor output will tend to drop to zero as the magnetic material goes into saturation, as shown in FIG. 3. Thus, looking at the output of an AMR sensor, one cannot tell if the device is in saturation.
What is needed, therefore, is a better approach to determining when an AMR sensor is in magnetic saturation.